Intensive screening promotes transpiration from the top

Reducing energy consumption in greenhouses is associated with higher humidity levels. Many growers are concerned that this could make the crop less active, impacting on transpiration from the top in particular. Last year, a Dutch research project was launched with the specific aim of measuring that. The conclusion: rather than a reason for concern, intensive screening is actually a way of improving the crop.
The “Transpiration from the Top” study took place at the Wageningen University & Research greenhouses in Bleiswijk, the Netherlands. Over two months, greenhouse climate and energy researcher Feije de Zwart tested a measurement method in a mature tomato crop and evaluated the quality of the technique. “We looked at the extent to which we could specifically measure transpiration at the top of a crop using a thermal imaging camera. And it worked. But this method requires an extremely accurate camera and a lot of attention. So for the time being, although this camera is excellent in the lab, it is not really suitable for use in the commercial setting.”

Measurement system

Low-energy cultivation not only requires better greenhouse insulation, but it also means the crop has to be grown in higher humidity levels. Growers tend to use their screens more often and don’t often open gaps in them. This increases the humidity in the greenhouse. “One of the reasons why growers are reluctant to accept this situation is that they are concerned the crop may not transpire enough,” de Zwart says. “After all, the more humid the air in the greenhouse, the lower the difference in vapour pressure between the crop and the greenhouse air, and the less the crop will transpire. They are most concerned about the top of the plant.”
These concerns stem from the fact that inadequate transpiration can affect nutrient transport to growing tips. He adds: “To measure is to know, so we developed a measurement system that can accurately determine transpiration at the top. A sensor of this kind could put growers’ minds at ease and could result in wider acceptance of higher humidity levels, particularly in vegetable cultivation.”

Camera plus artificial leaf

To test the system, the researchers installed a thermal camera above the crop. Besides the leaves on the crop itself, there were always two artificial leaves in the camera’s line of sight. One of these artificial leaves was fitted with a PT-100 temperature sensor to check the temperature registered by the camera. With this setup they were able to compare the temperature of a leaf at the top of the plant with that of an artificial leaf that was not transpiring, in the same conditions. The lower the temperature of the real leaf compared with the non-transpiring one, the greater the transpiration.
De Zwart again: “In fact, we noticed that the temperature of the real leaf fell quite a bit below that of the artificial leaf at night and that the temperature difference increased as the humidity level dropped. The best thing was that the behaviour of the real leaves was very much in line with our expectations based on our calculations. When we took another close look at the calculations, we noticed that the reduction in radiated heat loss brought about by screening really does increase the temperature at the top of the crop quite significantly. And this in turn leads to higher levels of transpiration at the top.”
De Zwart’s conclusion is therefore that transpiration at the top of the plant simply continues when screens are used, even if humidity is higher. “Intensive screening can limit transpiration from the crop as a whole but, conversely, stimulates it from the top of the crop.”

Vertical differences in the crop

These results could perhaps explain why good yields were achieved in all those practical trials with Next Generation Growing, despite the expectation that the high humidity would cause problems. De Zwart again: “If you look at water uptake, for example by comparing the amount irrigated and the drain, or by using a weighing gutter, you can barely see the effect of screening at all. But if you look at the increase in temperature in the crop, then you can see that closing the screen increases the temperature at the top, while often lowering it slightly further down in the canopy. This is because the use of the screen means less heating is needed further down.”
In any case, the temperature gradient across the crop drops, making transpiration more even throughout the crop. “That last factor, the evenness, had never really occurred to me,” he adds. “So rather than being a reason for concern, intensive screening is actually a way of improving the crop. This realisation is essentially the most important outcome of our research. It is such interesting information that it has been added to the Radiation Monitor.”

Radiation monitor

Growers and other interested parties who attended the Next Generation Growing course in the Netherlands will already be familiar with the Radiation Monitor. This online simulation model calculates the effects of screening and greenhouse covering materials on energy consumption and vertical temperature distribution.
De Zwart used the same model to establish the expected difference in temperature between a transpiring and a non-transpiring leaf. However, the data obtained from the project mentioned above demonstrated that the original calculation method was too inaccurate. Following improvements, the program now calculates transpiration at each layer of the crop.
The basis for this is that the difference in vapour pressure between the greenhouse air and the leaves plays a bigger role in driving transpiration than the local leaf temperature. The program can be used via the greenhouse horticulture models website.

“Impractical” method

De Zwart is slightly less enthusiastic about the results of the original project setup. “The measuring equipment was quite tricky to set up. We didn’t have a problem collecting images with a thermal camera, but focusing the lens was difficult. The plant was growing, so we had to constantly refocus the lens. Also, you have to use artificial leaves. We now know that a tomato leaf transpiring at the normal rate at the top of the plant is around 0.4°C cooler at night than a non-transpiring leaf in the same place.”
If this measurement method is used to distinguish normally transpiring leaves from leaves transpiring at a lower rate, the temperature differences measured should be in the magnitude of 0.2°C. These are such small differences that you would need to know exactly how warm a non-transpiring leaf would be in that position. That is why you need artificial leaves and a very accurate camera. He adds: “Actually, you do wonder whether the information you get is really worthwhile. After all, we now know that transpiration at the top of the plant simply continues when screens are used intensively, even if the air humidity is higher.”

Summary

Low-energy cultivation means a lot of screening hours and higher humidity in the greenhouse. Research shows that although higher humidity causes transpiration to decrease, the use of screens does not affect transpiration from the top of the plant. The project used a thermal camera and artificial leaves. The setup worked and, besides providing figures for the top of the crop, it also highlighted the vertical temperature distribution in the crop. The data was integrated into the online Radiation Monitor.